Chemistry: atomic structure and periodic trends (grade 10) · lesson

Protons, neutrons, and electrons

💡 The number of protons sets which element an atom is.

Protons (positive) and neutrons (no charge) sit in the tiny nucleus. Electrons (negative) move around it. A neutral atom has as many electrons as protons.

The atomic number is the number of protons. Every element has its own. Atoms can gain or lose electrons to become ions and still be the same element.

The mass number is protons plus neutrons. So neutrons = mass number − atomic number.

🧠 Let's work it out

Chlorine has 17 protons and mass number 35.

Neutrons = 35 − 17 = 18.
An atom with 11 protons is sodium no matter how many neutrons or electrons it has.

What do you think comes next?

atomic number: the number of protons in an atom
mass number: protons plus neutrons

🔗 PhET: Build an Atom · PhET Interactive Simulations (free) ↗

Isotopes and average mass

💡 Isotopes are atoms of the same element with different numbers of neutrons.

Carbon-12 and carbon-14 both have 6 protons, so both are carbon. Carbon-12 has 6 neutrons and carbon-14 has 8. The number after the name is the mass number.

The atomic mass on the periodic table is a weighted average of an element's isotopes. It is closer to the mass of the most common isotope.

🧠 How to think about it

Chlorine is 75% Cl-35 and 25% Cl-37. Multiply each mass by its share and add: 0.75 × 35 + 0.25 × 37 = 26.25 + 9.25 = 35.5.

isotope: a form of an element with a different number of neutrons

🔗 PhET: Isotopes and Atomic Mass · PhET Interactive Simulations (free) ↗

Valence electrons

💡 Outer (valence) electrons control how an atom bonds, and atoms in a group have the same number.

Valence electrons are in the outermost shell. They take part in bonding. Elements in the same column (group) have the same number, so they react in similar ways.

Group 1 has 1 valence electron and group 2 has 2. For groups 13 to 18, subtract 10 from the group number: group 16 has 6.

Metals tend to lose their few valence electrons. Magnesium (group 2) loses 2 to become Mg²⁺, because losing 2 is much easier than gaining 6.

🧠 How to think about it

Find the group, count valence electrons, then ask: is it easier to lose them or to gain enough to reach 8? That tells you the ion's charge.

valence electron: an electron in an atom's outer shell
ion: an atom with a charge because it gained or lost electrons

🔗 Khan Academy: Atomic models and periodicity · Khan Academy (free) ↗

Periodic trends

💡 Atoms get larger down a group and smaller across a period, because of shells and the pull of the nucleus.

Going down a group adds a new shell, so outer electrons sit farther from the nucleus. Potassium is larger than sodium.

Going across a period adds protons but no new shell. The stronger pull draws electrons in, so atoms get smaller and it takes more energy to remove an electron.

Group 17 atoms react by grabbing one electron. Fluorine is small, so its nucleus pulls hard on an incoming electron. That makes it more reactive than iodine.

🧠 Let's work it out

For any trend, ask two things: did a shell get added, and did the number of protons change?
More shells means bigger.
More protons in the same shell means a tighter pull.

What do you think comes next?

period: a row of the periodic table
group: a column of the periodic table